Vectors in Special Relativity

So for finding the magnitude of a vector, velocity for example, we use v=√(vx2+vy2+vz2), but in special relativity, velocities can not exceed c. Is their a different formula for magnitude in SR, or could a velocity like(in natural units) v=(.9,.9,.9) not exist, since the magnitude comes out to be about 1.5588c, which obviously exceeds c?